Thixotropic materials have the property of being fluid under shear but developing a gel structure and becoming self-supporting when at rest. The process is reversible. Thixotropic cements, which are thin and fluid during mixing and placement but reversibly form a rigid gel structure when pumping ceases, are useful in various oilwell applications. For example, thixotropic cement systems are used to plug lost circulation zones during both drilling and cementing operations, to repair damaged or corroded casing, as grouts, and to limit annular gas migration in some situations. Such systems have also been used in situations where weak formations are exposed and would otherwise fracture under the hydrostatic pressure of a cement column: with a thixotropic cement the hydrostatic pressure of the column diminishes as the cement gels.
Various thixotropic cement systems are known in the prior art, including the following:
1. Clay-based systems. These typically comprise Portland cement and bentonite clay. PA0 2. Calcium sulphate-based systems. The material most widely used for this purpose is calcium sulphate hemihydrate. See, for example, U.S. Pat. No. 3,847,635. PA0 3. Aluminium sulphate/iron (II) sulphate systems. See, for example, U.S. Pat. No. 4,415,367. PA0 4. Crosslinked cellulose polymer systems. See, for example, U.S. Pat. Nos. 3,959,003, 3,804,174 and 4,524,828. PA0 5. Mixed metal hydroxide systems. See, for example, U.S. Pat. No. 4,822,421. PA0 1. Cementitious materials, e.g. cements, particularly Portland cements, blast furnace slag, fly ash/lime mixes and mixtures of these materials. PA0 2. Other ceramic-forming materials. PA0 3. Polymeric materials, e.g. thermosetting polymers etc.
The known thixotropic cement systems have various limitations, one of which is that a significant time is taken for a gel structure to develop on removal of shear: at best this is of the order of several minutes and can be substantially longer, possibly approaching the timescale over which the cement sets. This can present problems in certain situations.